1. Field of the Invention
The present invention relates generally to sealing systems for thermoplastic pipes and, specifically, to an improved belling apparatus and process for installing a gasket in a socket end of a thermoplastic pipe.
2. Description of the Prior Art
Pipes formed from thermoplastic materials including polyethylene and PVC are used in a variety of industries. In forming a joint between sections of pipe, the spigot or male pipe end is inserted within the female or socket pipe end. An annular, elastomeric ring or gasket is typically seated within a groove formed in the socket end of the thermoplastic pipe. As the spigot is inserted within the socket, the gasket provides the major seal capacity for the joint. It is critical, during the installation process, that the gasket not be able to twist or flip since a displaced or dislocated gasket will adversely affect the ultimate sealing capacity of the joint.
A variety of other shortcomings have existed in such pipe joints of the prior art. For example, unintentional earth loading, such as traffic load, can be transferred to the socket end of the pipe sections, leading to deformations and associated leakage in the joints. Certain heat strains in the pipes, such as can occur on storage in hot weather or by close proximity to heat sources can result in deformations, shrinkage of the pipe material and resulting leakage.
One early attempt to ensure the integrity of such pipe joints was to provide local reinforcement of the groove portion of the socket end by means of a heavier wall thickness in this region of the pipe. In some cases, reinforcing sleeves or external connecting mechanisms were also utilized. Each of these solutions was less than ideal, in some cases failing to provide the needed joint integrity and often contributing to the complexity and expense of the manufacturing or field assembly operations.
In the early 1970's, a new technology was developed by Rieber & Son of Bergen, Norway, referred to in the industry as the “Rieber Joint.” The Rieber system employed a combined mould element and sealing ring for sealing a joint between the socket end and spigot end of two cooperating pipes formed from thermoplastic materials. In the Rieber process, the elastomeric gasket was inserted within an internal groove in the socket end of the female pipe as the female or belling end was simultaneously being formed. The provision of a prestressed and anchored elastomeric gasket during the belling process at the pipe factory provided an improved socket end for a pipe joint with a sealing gasket which would not twist or flip or otherwise allow impurities to enter the sealing zones of the joint. These features increased the reliability of the joint and decreased the risk of leaks or possible failure due to abrasion or other factors. The Rieber process is described in the following issued U.S. Pat. Nos. 4,120,521; 4,061,459; 4,030,872; 3,965,715; 3,929,958; 3,887,992; 3,884,612; and 3,776,682.
Certain of the polyolefin materials were not well adapted for use in the Rieber-type process. Polyethylene, for example, after being heated has a tendency to return to its original shape. Thus, after a bell connection is formed using the traditional Rieber techniques of heating, forming and cooling, the end result is a shape that is not stable.
Accordingly, an object of the present invention is to provide a method of installing a gasket in a socket end of a thermoplastic pipe, including a polyethylene pipe, in a Rieber-type process which provides a stable shape upon cooling.
Another object is to provide such a manufacturing technique which is simple to implement and which provides reliable results.
Another object of the invention is to provide an integral gasket within the socket end of a polyethylene pipe which is securely retained within a receiving groove which is integrally formed about the gasket during the manufacturing step.
Another object of the invention is to allow the manufacturing of very large diameter gasketed pipe at a lower cost that traditional methods.